Punched components of the type indicated above and which are provided with a polymer film as a protective layer are used, for example, as entrance sills for motor vehicles. They are subjected to a high degree of wear due to the effects of weather and constant stress. In particular, corrosion may occur on the punched surfaces and in the cross-over regions between the metal sheet, which can be made, for example, of aluminium or also of a Cr—Ni stainless steel, and the polymer film. In particular, crevice corrosion and filiform corrosion often occur.
Methods for manufacturing this type of punched component, which has a surface coating in the form of a polymer film to protect against corrosion, are known, for example, from WO 96/25292. The polymer film is provided here on its upper side with a scratch-resistant layer which serves to protect against corrosion. On the lower side of the polymer film which has the metal sheet a layer of hot melt adhesive is provided which is thermally activated upon laminating the polymer film onto the metal sheet.
The methods known from the prior art for manufacturing a punched component have proven to be of value. However, it is considered to be a disadvantage that cracking occurs during the punching process if the polymer film can not follow the deformations of the metal substrate. In order to prevent cracking, the entire structure of the coatings must follow the deformations of the metal sheet. Until now, one has not been able to totally guarantee this in practice because, for example, small discrepancies in the anchoring of the polymer film may lead to the polymer film being re-set, and so may lead cracking.
In particular when using hot-melt adhesive, the risk of cracking is increased because the punching process takes place at ambient temperature and hot melt adhesive becomes brittle in this temperature range.
Moreover, with a brushed metal surface an increased risk of cracking is observed because the metal surface has a distinct fine structure due to the brushing process. The large number of microscopically small openings between the polymer film and the metal surface are weak points which may lead to cracking with corresponding stressing of the punched edge. This in turn leads to crevice corrosion if a crevice with a width of between 0.02 mm and 0.5 mm forms, and due to lack of oxygen passivation of the metal surface can not take place. Crevice corrosion occurs with almost all metals, including Cr—Ni stainless steels. Moreover, crevice corrosion also occurs with crevices which form between metal and plastic.
With crevice corrosion, due to the high concentration of oxygen a cathodic reaction takes places at the start of the crevice, while at the end of the crevice an anodic reaction that removes metal occurs.
Filiform corrosion is manifested by thread-shaped infiltrations between the coating and the metal surface. With aluminium, this type of corrosion, which corresponds electrochemically to crevice corrosion, is a frequently observed cause of damage.